Two distinct cannabinoid receptors have been identified in mammalian tissues: cannabinoid (CB1) receptor is mainly located in the central nervous system, whereas CB2 receptor is primarily expressed by immune and tumor cells.
The high level of CB2 receptor expression in immune cells and much lower expression in other cell types, particularly in the CNS, makes this receptor an attractive target for imaging and monitoring of therapy for neurological diseases. Specifically, CB2 receptor expression is high in spleen, tonsils and thymus, and low—or even undetectable—in brain, thyroid, retina, placenta, skeletal muscle, kidney, liver, adrenal gland, heart, prostate and ovary. This expression profile provides great opportunities for imaging with low background. Furthermore, CB2 receptor expression is highly plastic and may be induced under specific disease conditions, for example in tumor cells and CNS-resident microglia. Accordingly, CB2 receptor has become a predominant target for drug development aimed at treating pain, chronic inflammation, osteoporosis, malignant gliomas, tumors of immune origin and immunological disorders, and thus developing tool that allow for its precise mapping in tissue and for identifying novel CB2 receptor specific ligands is desired.
Together with the characterization of the CB2 receptor, a considerable effort has been made to develop CB2 receptor ligands. The term cannabinoid was first used to describe terpenophenolic compounds in Cannabis Sativa L., among which (−)-trans-Δ9-tetrahydrocanabinol (Δ9-THC) is the main bioactive constituent. Many anti-inflammatory effects of Δ9-THC have been described, including inhibition of tumor necrosis factor-α, interleukin-2, nitric oxide and arachidonic acid production from macrophages and T cells,
CB2 receptor ligands can be divided into three main groups: plant-derived, endogenous and synthetic. The best-known plant-derived cannabinoid is Δ9-THC, but cannabinol and cannabidiol also induce profound biological effects. Two endocannabinoids, arachidonoylethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG), have been identified.

Both molecules above have greater affinity at CB1 than CB2 receptors. 2-AG acts as a full agonist at CB1 and CB2 receptor, and anandamide acts as a partial agonist (showing mixed agonist-antagonist properties) toward these receptors. Many synthetic cannabinoid receptor ligands have been developed, including HU-210, CP55940, WIN55212-2, SR141617A, AM630 and SR144528. HU-210, CP55940, WIN55212-2 are cannabinoid receptor agonists with no or marginal CB1/CB2 selectivity. AM630 and SR144528 are both selective CB2 receptor ligands and behave as inverse agonists rather than “silent” or “neutral” antagonists.

The CB2/CB1 affinity ratio is less for AM630 (CB2/CB1 affinity=165) than for SR144528 (CB2/CB1 affinity>700). Accordingly, SR144528 has been widely used as a pharmacological tool to determine CB2 receptor-mediated effects. However, the use of SR144528 for CB2 receptor-targeted imaging has never been tested directly since SR144528 is not conjugable. In other words, signaling moieties, such as fluorescent dyes, lanthanide chelates and nanoparticles, cannot be easily coupled to SR144528. Thus, to further study CB2 receptor and diseases associated with an increase in the expression of this receptor, development of a conjugable SR144528 analog constitutes an essential step.
CB2 receptor has become an attractive target for imaging and treating pain, inflammation, osteoporosis, diseases of the liver, growth of malignant gliomas and tumors of immune origin and immunological disorders such as multiple sclerosis. SR144528 is a selective CB2 receptor ligand with high CB2/CB1 affinity ratio (>700). Embodiments of the present invention couple moieties to a conjugable form of SR144528, to provide opportunities in CB2 receptor targeted disease imaging and to screen for novel CB2 receptor ligands. Thus aspects of the present invention is a conjugable form of SR144528 with six carbon spacer (C6SR144528 or mbc94) and, as one embodiment of the present invention, it is coupled to a near-infrared dye, IRDye™ 800CW NHS ester, for CB2 receptor targeted imaging.